A block diagram of a typical state of the art weather radar is shown in FIG. 1 and will include a transmitter 10 comprising a pulse modulator coupled with a magnetron. The pulse modulator essentially modulates the magnetron causing same to emit a short burst of microwave energy corresponding to the transmitted pulse. These pulses are coupled through a four port circulator 11, that permits the transmitted pulses to go to antenna 12 without coupling same to the receiver 13. However, when the return pulses are received by the antenna, the four port circulator permits reception of the received pulses by the receiver 13 and will not deliver the pulse to the transmitter 10. In any event, the transmitted pulse is radiated by the parabolic dish portion of the antenna 12 generally from the nose of an aircraft toward possible weather. The received reply will be indicative of the presence of rain in cloud formations in the direction the antenna is pointing.
Once the four port circulator delivers the received pulses to the receiver, the information is detected into a low frequency video signal and transmitted to the indicator 15 on the "video" line. Indicator 15 also takes trigger information indicative of the starting of the transmitter pulse from transmitter 10 on the trigger pulse line 14. This trigger pulse informs the indicator as to the instant in time of pulse transmission so that the indicator can gauge time with respect to same for calculating range, etc. Furthermore, the indicator couples the detected return pulses with the trigger pulses and azimuth information from the antenna thereby providing a visual indication of the antenna scanning position with respect to azimuth.
The subject invention, while pertaining generally to weather radars, is more particularly directed to the pulse modulator portion of the associated transmitter. Known prior art radar pulse modulators generally fall into three basic types: e.g., line-type, magnetic and active switch. The line-type and magnetic modulators use pulse forming networks (PFN) coupled with a charging circuit and a switching element to drive the load (a magnetron or other microwave RF tubes). The shape and duration of the pulse produced by modulators is determined by passive circuit elements which present severe design and manufacturing limitations in that the pulse repetition frequency (PRF) corresponds to reset and charging time. Further, the pulse duration cannot be readily changed (long pulse durations require large PFNs) and pulse flatness and ripple are generally hard to control. The use of resonant delay lines in combination with saturating step up transformers (in addition to being difficult to control) are expensive and require a high degree of skill in manufacturing and assembling.
As will be described, the subject modulator is non-resonant and capable of being constructed of relatively low cost generally off the shelf items that do not require difficult and time consuming assembly techniques. The nature of the operative environment (weather radar) suggests the utilization of certain components such as magnetrons for the RF pulse producing circuit. Accordingly, the utilization of a transistor switch and a step up transformer for driving a magnetron along with the associated circuitry comprise an important feature of the subject invention.
The subject invention, including the transistorized active switch pulse modulator, provides superior performance over known prior art devices in that the pulse repetition frequency and pulse duration can be readily changed. Very long RF pulses are now limited only by storage capacitance, transformer saturation and associated magnetron limitations. Further, the pulse ripple will be quite low and the pulse slope is automatically regulated by a unique feedback network in order to maintain the desired RF pulse frequency spectrum.
One of the principal objects of the invention is to provide a uniquely constructed circuit and method for producing pulse modulation in a weather radar system for aircraft.
Another object of the invention is to provide a uniquely constructed transistorized active switch pulse modulator in which the pulse repetition frequency (PRF) and pulse duration can be readily changed. It is a feature of the invention that long pulses are limited only by storage capacitance, transformer saturation and associated magnetron limitations.
A further object of the invention is to provide a pulse modulator of the character described including a unique circuit means for controlling a transistorized switch to thereby cause the voltage source drive to a magnetron to appear similar in nature to a functionally desirable current source drive.
A further object of the invention is to provide a pulse modulator of the character described which utilizes a magnetron wherein a pulse ripple is quite low and wherein pulse slopes are regulated by a unique feedback loop to maintain a desired sinusoidal frequency spectrum.
A still further object of the invention is to provide a transistorized switch with a step up transformer driving a magnetron in the pulse modulator for weather radar. It is a feature of the invention that the modulator does not incorporate the use of critically tuned elements such as pulse forming networks and matching transformers. Further, should the magnetron fail to oscillate (fire) on a given pulse thus appearing as an open circuit, the applied voltage will not double as in other forms of pulse modulators and the danger of arcing throughout the pulse modulator is thereby obviated.
Another object of the invention is to provide a unique circuit means for protecting circuit components utilized in the pulse modulator portion of weather radar equipment. It is a feature of the invention that large current build ups caused by arcing in the magnetron are prevented by effective use of this circuit means.